In semiconductor fabrication, “missing metal” defects, electromigration, and stress migration are issues that often occur after chemical-mechanical polishing. The effects and severity of the issues strongly depend on the post-electrochemical plating (ECP) copper film grain size, orientation, and intrinsic stress in patterned structures. Deformed and highly stressed crystals are transformed into unstressed crystals by recovery, recrystallization and grain growth. During the recrystallization process, the sheet resistance (Rs) typically drops 20 to 30%, attaining a sheet resistance approaching that of pure, bulk copper. Copper grain size, orientation, film stress, and sheet resistance also continuously change after the ECP process due to stabilization of vacancies, dislocations, and other crystalline imperfections and impurities. The final copper film characteristics also depend critically on the plating process, bath chemistry, and anneal process conditions.
To determine potential electrical and reliability problems, it is therefore useful to analyze the microstructure characteristics of the copper film. Typically, copper film grain size characterization efforts have been performed using electron back scatter diffraction (EBSD), transmission electron microscopy (TEM) imaging, or by focused ion beam (FB) imaging. Usually 1000 grains per sample are traced manually, the tracing is digitized, and the grain areas are measured by image analysis software. The measured areas are then converted to an equivalent grain diameter. Such measurement techniques, however, need to be performed off-line, are time-consuming, and significantly limit fast process characterization, making copper grain boundary monitoring impractical. In addition, further analysis of the grain size distribution involves consideration of at least two parameters (average and variance), thus complicating the characterization.
Accordingly, there is a need for a system and method for performing in-line and efficient grain size characterization that overcomes the above problems.